1. Field of the Invention
This invention relates to a method of separating liquid mixtures. More particularly, it relates to an industrially advantageous method of separating liquid mixtures by pervaporation which comprises supplying the feed liquid mixture in the form of a mixed fluid at least part of which is in the form of a vapor to a membrane separator for direct contact of said vapor with a liquid film occurring on a permeation membrane and consisting of said liquid mixture to thereby cause partial condensation of said vapor in said liquid film so that the temperature decrease in the liquid mixture on the permeation membrane can be reduced.
2. Description of the Prior Art
The so-called pervaporation which comprises separating a mixture of liquids by feeding the liquid mixture to one side of a permeation membrane, while maintaining the opposite side of said membrane under reduced pressure by connecting to a vacuum source or while maintaining said opposite side at a lower partial vapor pressure by passing an inert gas, to thereby cause liquid permeation under the resulting pressure gradient and evaporation of the permeate liquid on the lower pressure side has been a subject of study since the mid-1950's. This separation method has been devised for the separation and/or purification of those liquids (mostly, organic solvents, hydrocarbons, etc.) which cannot be fractionated by ordinary distillation methods. Known examples of its application include fractional separation of azeotropic mixtures, solvents close in boiling point, isomers (ortho and para, cis and trans) and the like, concentration and/or purification of heat-degradable liquid mixtures or juices, removal of trace impurities, and removal of byproduct water during esterification reactions [for example, U.S. Pat. No. 2,953,502; C. Y. Choo, "Advances in Petroleum Chemistry and Refining", vol. VI (1962), page 72; U.S. Pat. No. 2,956,070].
As mentioned above, pervaporation is a method of separation which invariably involves the liquid-to-gaseous phase change. The heat (latent heat) required for said phase change comes from the sensible heat of the liquid mixture fed to the separation apparatus. Therefore, the liquid temperature in the separation apparatus gradually decreases. The greater the quantity of the permeant liquid, the more significant the temperature decrease is.
On the other hand, it is known that the membrane separation efficiency in pervaporation is highly temperature-dependent. Thus, generally, when the temperature is lowered, the quantity in which the permeant can permeate the membrane decreases. When, conversely, the temperature rises, the separability (separation factor) tends to become worse. As a result, there is an optimum separation temperature range for each liquid mixture and, moreover, the range is not so wide. Therefore, it is important in practical use of pervaporation to operate such that the liquid temperature difference in the membrane separator (hereinafter referred to as module unit) will not become too great.
For solving such problems, the use of a number of module units with a heater disposed before each module unit, namely a multistep system, has been devised and tried [for example, Takashi Ishikawa, Chemical Engineering, 29 (6), 19 (1984); Takashi Ishikawa, Kagaku Sochi (Chemical Apparatus), 25 (12), 27 (1983)]. An example of such multistep system, which includes 4 module units, is schematically shown in FIG. 2. In the system shown in FIG. 2, a liquid mixture 10 is introduced into a first heater 11 and heated there to a temperature below the boiling point thereof and then introduced into a first module unit 12 at one end thereof. The nonpermeated portion 14, which has not permeated through the permeation membrane 13, is introduced into a second heater 15. Thereafter, in the same manner as mentioned above, said portion 14 from the first module is heated and then introduced into a second module unit 16, the non-permeated portion 18 which has not permeated through the membrane 17 is introduced into a third heater 19 and then into a third module unit 20, and the non-permeated portion 22 which has not permeated through the membrane 21 is introduced, via a fourth heater 23, into a fourth module unit 24. The non-permeated portion 26 which has not permeated the membrane 25 in the fourth module unit is one product. Another product is obtained by combining the vapors of the permeated liquid portions (permeates) which have permeated through the membranes 13, 17, 21 and 25 in the first to fourth module units, respectively, and recovering the resultant permeated fraction 27 or by recovering individually the permeate fractions 28, 29 30 and 31 respectively obtained from said vapors. As mentioned above, in multistep apparatus, heaters and module units are connected in series to form two or more heater-module unit sets (or steps), and the heat to be required in each module unit is supplied in the preceding heater, hence the whole heat required is supplied dividedly in portions.
However, a considerable number of steps are required for the separation temperature to be uniform in the above-mentioned multistep apparatus, and this renders the apparatus complicated. Therefore, 2 to 5 steps are generally used. Under these circumstances, the problem of making the separation temperature uniform remains unsolved even in such multistep systems as far as those cases in which the quantity of permeant liquid is great are concerned.